The present invention relates to novel modified pigment grade strontium chromate corrosion inhibitors characterized by reduced solubility in water and undiminished CrO4xe2x88x92xe2x88x92content.
Several pigment grade inorganic and organic chromates are known to the art and employed in organic based protective primer applications, most notably: SrCrO4 and Ca++, Ba++, Zn++chromates.
Many chromate corrosion inhibitor pigments used to be ubiquitously applied in the paint and coating industry, worldwide as well as in the United States. Due to environmental concerns, however, most of them have been practically eliminated from general industrial use during the past two decades and replaced with less effective phosphate-, molybdate-, borate- or silicate-based products, which, notably, also represent all of the available possibilities offered by inorganic chemistry for this purpose. The elimination of chromate inhibitor pigments from the paint and coating industry appears to be a compromise allowed primarily by improvements in coating science and resin chemistry. Notable exceptions are high-performance organic coatings, especially aircraft and coil primers, which possess no significant barrier capacity and consequently, require the inhibitor efficiency of chromates without any possibility of compromise. Not surprisingly, the corrosion inhibitor pigment of choice for aircraft and coil primers is SrCrO4, a preference entirely justified, considering this product""s remarkably advantageous pigment qualities, as subsequently discussed.
Corrosion inhibitor pigments function as reservoirs of inhibitor species built into organic primers. In general, anions are the active inhibitor species of corrosion inhibitor pigments, which, in the case of chromates, is CrO4xe2x88x92xe2x88x92. It will be apparent, that high CrO4xe2x88x92xe2x88x92 concentration per unit volume of pigment, i.e., high xe2x80x9cspecific inhibitor capacityxe2x80x9d, is the desirable property of pigment grade chromates. This is particularly true in view of the fact the average thickness of high performance aircraft and coil primers is approximately 10 to 20 microns.
Understandably, high inhibitor capacity is expected to be concentrated in the limited volume available for corrosion inhibitor pigments in high performance organic primer applications. This concept is quantified by
eisp=cixc2x7gsp/wm.
eisp represents the specific inhibitor capacity of corrosion inhibitor pigments, where Ci and gsp are, respectively, the weight ratio of CrO4xe2x88x92xe2x88x92 in the pigment and the specific gravity of the pigment, and Wm is the molar weight of the inhibitor species, in this case of CrO4xe2x88x92xe2x88x92=116. It will be apparent that a combination of high CrO4xe2x88x92xe2x88x92content and high specific gravity results in high specific inhibitor capacity, as it can be seen by comparing the cases of CaCrO4 and SrCrO4 presented in Table 1. Due to higher specific gravity, the specific inhibitor capacity of SrCrO4 (18.4 mmol/cm3) is higher at only 57% CrO4xe2x88x92xe2x88x92 content than at 74% CrO4xe2x88x92xe2x88x92 content of CaCrO4 (eisp=13.1 mmol/cm3). Specific inhibitor capacity is not informative, however, with respect to inhibitive activity of corrosion inhibitor pigments. Corrosion and its inhibition are aqueous, dynamic processes and consequently, in coating applications, the kinetic availability of inhibitor species is essentially a function of the solubility of pigment grade corrosion inhibitors. It will be apparent that corrosion inhibitor pigments must possess effective, however limited, solubility in water, as well as to dissociate and hydrolyze as ordinary electrolytes:
SrCrO4⇄Sr+++CrO4xe2x88x92xe2x88x92+H2O⇄HCrO4xe2x88x92+OHxe2x88x92xe2x80x83xe2x80x831.
The corrosion inhibitor activity of pigments can be conveniently qualified by Ii=nxc2x7cisat/cicrt, the inhibitive activity parameter, where cisat and cicrt are the is pigments"" solubility and, respectively, the critical concentration of the related inhibitor species, in this case CrO4xe2x88x92xe2x88x92. A stoichometric parameter, n=1 in the case of alkaline earth chromates. As for cicrt, it characterizes distinct inhibitor anionic species and it is defined as the minumum concentration of anionic species necessary to maintain metal substrates in a passive state when in contact with aqueous phases. It will be apparent that Ii less than 1 indicates low inhibitor activity of pigments and consequently, are unacceptable materials. Similarily, high values of Ii are not acceptable, because highly soluble pigments promote degradation of organic coatings by osmotic blistering as well as by leaching, the latter resulting in increased porosity and increased water absorption in organic coatings. Consequently, it appears that organic coatings impose a limited window or range of solubilities for corrosion inhibitor pigments. This range is estimated to correspond to 1 less than Ii less than 100 and preferably 1 less than Ii less than 10. As can be seen from Table 1, SrCrO4 is characterized by Iixcx9c5 and corresponding solubility of about cisat=5 mmoles/L.
As can be seen, due to its high specific inhibitor capacity (eisp=18.6 mmol/cm3, the highest of all corrosion inhibitor pigment grade chromates) and with moderate yet effective solubility (Iixcx9c5), SrCrO4 appears to be the most effective and valuable of all chromate corrosion inhibitor pigments. However, due to its solubility, SrCrO4 is also moderately hygroscopic, an undesirable property of some characteristic finely divided pigment grade products. This generally results in unwanted water vapor absorption from the surrounding air, and consequently, a moderate tendency for agglomeration or clump formation, especially during transport or storage in high humidity conditions. Although clumping does not necessarily alter the initial particle size distribution or fineness of pigment grade SrCrO4, it is undesirable, nevertheless, for it adversely affects dispersion procedures and ultimately results in increased energy consumption in paint manufacture.
As intuitively expected, the kinetics and extent of the water absorption process by pigment grade SrCrO4 is a function of the relative humidity (R.H.) of the surrounding air, among other factors. At 60% R.H., the extent of water vapor absorption by SrCrO4 is negligible and independent of exposure time.
In contrast, however, the rate of water vapor absorption is considerable at 100% R.H. and a function of exposure conditions and exposure time. Such conditions can result in a significant increase (4-7% by weight) in the moisture content of pigment grade SrCrO4. As also expected, the process is found to be reversible. It is observed, that water absorbed by SrCrO4 at 100% R.H., desorbs spontaneously at 60% R.H. and ambient temperature, until a corresponding equilibrium value of  less than 0.1% moisture content is established. It is apparent that clumping of pigment grade SrCrO4 is a consequence of the water absorption process"" reversible character and, presumably, it specifically occurs during desorption.
It is a principal object of the present invention to provide a process and a strontium chromate, calcium chromate or zinc (II) chromate product produced by the process having reduced solubility in water with undiminished CrO4xe2x88x92xe2x88x92 content. It is a further aspect of the invention that the strontium chromate products display noticeably enhanced corrosion inhibitor performance. In accordance with a further aspect of the invention, the products have a reduced tendency for slump formation.
It was discovered pursuant to the present invention, that the natural solubility of chromates such as SrCrO4 in water (about 5 mmole/L at ambient temperature) can be reduced approximately 40% to 70%, without any negative effect on the pigment""s corrosion inhibition performance. To the contrary, as documented in Example 6, pigment grade SrCrO4 of reduced solubility produced, according to the present invention, displays incrementally but nevertheless noticeably enhanced corrosion inhibition performance in organic coating applications.
It appears that the natural solubility of SrCrO4, although moderate (see Table A), is excessive to some extent in comparison to an optimum solubility value. The above consideration is also supported by Ii=5 of SrCrO4 (see Table A), indicating that its cisat is about 5 times higher than cicrtxcx9c1 mmol/L of CrO4xe2x88x92xe2x88x92, that is the minimum concentration required for maximum possible corrosion inhibitor performance of the species. Considering all of the above, it appears reasonable to state that the solubility of SrCrO4xe2x88x92xe2x88x92(or generally, of any pigment grade corrosion inhibitor containing CrO4xe2x88x92xe2x88x92 species), should be: 1 mmol/L less than cisat less than 5 mmol/L.
In addition to noticeable enhancement of its corrosion inhibitive performance, additional beneficial effects of solubility reduction of pigment grade SrCrO4 are: improved correlation between pigments"" solubility and paint stability as well as reduction of leaching of related ionic species from organic coatings.
It is well known with regard to paint stability, that ionic species such as CrO4xe2x88x92xe2x88x92 and Sr++ dissolved in the aqueous phase of water-born paint formulations, adversely affect the Theological stability of latter. In general, direct proportionality can be expected between the solubility of pigments, or concentration of related species, and the extent of this de-stabilizing effect. Consequently, reduction of solubility of pigment grade SrCrO4 should be beneficial in this respect. Similarly, the leaching rates of CrO42xe2x88x92 species from organic coatings are directly proportional with cisat of related pigments, in this particular case with the solubility of SrCrO4. Leaching, while useful due to its being the only transport process for inhibitor species at discontinuity sites in organic coatings, is also detrimental, however, due to causation of increased porosity and, in general, in diminished protective capacity of pigmented primers.
Moreover, it has been observed, that reduced solubility, realized according to the process of the present invention, results also in significant reduction of the clumping tendency of pigment grade SrCrO4.
A direct correlation was intuitively expected between the solubility (moderate hygroscopic character) and clumping tendency of SrCrO4. Indeed, it has been observed, that reduced solubility, realized according to the present invention, results also in significant reduction of the clumping tendency of pigment grade SrCrO4. This observation is documented in Example 7.
Considering all of the foregoing, it appears desirable to optimize the solubility of corrosion inhibitor pigments, in general. In the particular case of SrCrO4, it appears that by reducing solubility, all functional characteristics of this pigment can be improved, inclusive of corrosion inhibitor performance and rheological behavior. As specified earlier however, cisat or solubility of SrCrO4 must not be reduced to less than 1 mmol/L.
In accordance with the invention, a procedure has been found which is very effective in reducing the natural solubility of SrCrO4. This procedure is based essentially on a chemisorption process, which occurs spontaneously on the surface of finely divided SrCrO4, when the latter is dispersed in an aqueous solution of any one or more of a diverse group of soluble salts or derivatives of phosphoric acid.
An important aspect of the invention is the use of derivatives of H3PO4 as surface modifying agents for treating strontium chromate and altering the surface characteristics thereof. In accordance with a further aspect of the invention, the derivatives of phosphoric acid used to modify the strontium chromate surface characteristics include alkali metal salts, alkali metal acid salts, and ammonium salts phosphoric acids including orthophosphoric acid, metaphosphoric acid and phosphorus acid.
In accordance with an important aspect of the invention, preferred surface modification agents for treatment of strontium chromate include alkali, alkali acid salts, or ammonium salts of acids selected from the group consisting of H4P2O7, H5P3O10, H3PO3 and H3PO4.
In accordance with a further related aspect of the invention, such preferred treating agents include materials selected from the group consisting of: Na4P2O7, K4P2O7, Na5P3O10, K5P3O10, Na2HPO3, K2HPO3, Na3PO4, K3PO4, NaH2PO4, KH2PO4, Na2H2P2O7, K2H2P2O7, (NaOPO2)3, (KOPO2)3, NaNH4HPO4.4H2O and KNH4HPO4.4H2O.
As illustrated below for Na4P2O7 (tetra-sodium pyrophosphate), one of the preferred surface modifying agents, the chemisorption of diverse ionic phosphate species occurs, presumably, by an exchange mechanism, with the participation of the anionic species: SrCrO4/(2SrCrO4)n+nP2O74xe2x88x92⇄SrCrO4/(Sr2P2O7)n+2nCrO4xe2x88x92xe2x88x922. where typically n=10xe2x88x924-10xe2x88x922.
It is reasonable to assume, that chemisorption of diverse ionic phosphate species results in formation of essentially insoluble compounds on the surface of treated SrCrO4, and consequently, the equilibrium of Reaction 2 is shifted to the extreme right. The resultant surface species are believed to be chemically similar to the correspondent bulk salts of strontium, such as Sr2P2O7 or strontium pyrophosphate, in the above case.
Taking into account the above considerations, it appears reasonable to assume that chemisorption of phosphate species radically alters the chemical composition of the surface of SrCrO4 crystallites, and as a consequence, alteration of some of related physical surface properties can be intuitively expected.
An important aspect of the present invention is that chemisorption of diverse phosphate ionic species results in reduction of solubility of SrCrO4 and as a consequence, in improvement of all functional characteristics of this pigment grade product, in particular, its corrosion inhibitor performance and mitigation of its tendency for clump formation. It has been observed that the chemisorption process (Reaction 2), occurs practically instantaneously.
A function also of the SrCrO4 phase""s specific surface area, a critical stoichiometrical ratio apparently exists between the treating species and SrCrO4, above which the process is self-limiting, as might be expected on basis of generally accepted concepts of surface chemistry.
For example, in the exemplary case of P2O74xe2x88x92, at typical values of specific surface area of SrCrO4 corresponding to a 2-5 micron average particle diameter, the critical value of the molar ratio was found to be in the range of Na4P2O7/SrCrO4=9xc3x9710xe2x88x924-1.8xc3x9710xe2x88x923 above which the chemisorption process is self-limiting.
The self-limiting character of the chemisorption process indicates, that at the critical value of the molar ratio, the adsorbent solid phase""s particle surfaces become completely covered by a layer of chemisorbed species, and, essentially, Reaction 2 stops. Due to the practically insoluble character of the chemisorbed Sr2P2O7 layer, it was observed that the chemisorption process becomes self-limiting even at the quite low initial bulk concentrations of about 0.5-1.5 mmole P2O74xe2x88x92/L.
As for distribution, it is reasonable to assume a uniform spread of the chemisorbed layer on the adsorbent""s surface, although the process could preferentially occur at high energy sites such as edges or tips of crystallites. Assuming, however, an insoluble layer of Sr2P2O7 uniformly distributed on the surfaces of pigment grade SrCrO4, of particles with diameters of 5 micron, the thickness of insoluble layer (at the molar ratio value of Sr2P2O7/SrCrO4=1xc3x9710xe2x88x923) is estimated at approximately 10-20 Angstrom. It will be apparent, however, that in the 0 less than Sr2P2O7/SrCrO4 less than 1xc3x9710xe2x88x923 value range of molar ratios, the corresponding thickness of the chemisorbed layer will be 0-10 Angstrom and the adsorbant""s surface coverage, only partial.
The chemisorbed layer may represent 0.005% to 1% of the modified chromate. Generally, the chemisorbed Sr2P2O7 layer, at saturation, amounts to about 0.017-0.34% by weight of the resultant surface modified SrCro4, and consequently some of the desirable physical characteristics of the latter, such as specific gravity, pH or color, are unaltered. As will also be apparent, the CrO42xe2x88x92 content of surface modified SrCrO4 produced according to the present invention, is substantially undiminished in comparison to the pigment grade versions heretofore known in the art.
Understandably and with no intent, however, to limit the concept, the preferred examples of realization of the present invention are versions of pigment grade SrCrO4 surface modified to saturation, which can be formally symbolized by the general formula of (Srx/2PyOz)n/SrCrO4, where n, the stoichiometric coefficient is approximately n=1xc3x9710xe2x88x924-2xc3x9710xe2x88x923. Srx/2PyOz symbolizes chemisorbed layers consisting of Sr-salts of various oxy anions of phosphorus, organic derivatives and hetero-poly anions included, and x,y,z are correspondent stoichiometric indices. In the significant case of P2O74xe2x88x92as a preferred chemisorbed species, the resultant product is symbolized by (Sr2P2O7)n/SrCrO4, where n=1xc3x9710xe2x88x924-2xc3x9710xe2x88x923.
Notably, the chemisorbed layers obtained in accordance with the present invention, possess remarkable mechanical stability, which assures the feasibility of the surface modification procedure in industrial conditions. For example, it was observed, pursuant to the present invention, that the Sr2P2O7 layer of (Sr2P2O7)n/SrCrO4, n=1xc3x9710xe2x88x924-2xc3x9710xe2x88x923, withstands, without apparent breakdown, drying, and, more specifically, subsequent grinding, which are some of the typical steps used in pigment manufacture.
As expected on the basis of Reaction 2, chemisorbed layers on SrCrO4, obtained according to the present invention, also display stability in aqueous media in neutral, moderately acidic or alkaline conditions, wherein they resist dissolution. It was learned, however, that the stability of freshly formed chemisorbed layers is limited. They degrade on extensive contact, for several hours at elevated temperature with aqueous phases. Notably, it was also realized pursuant to the present invention, that chemisorbed layers on SrCrO4 must be effectively stabilized by drying of the previously filtered products, typically performed at 100-110xc2x0 C. For example, (Sr2P2O7)n/SrCrO4, n=1xc3x9710xe2x88x924-2xc3x9710xe2x88x923 produced according to the present invention, is processed by filtration and subsequent drying at 100-110xc2x0 C.
Stabilized Srx/2PyOz layers of (Srx/2PyOz)n/SrCrO4, n=1xc3x9710xe2x88x924-2xc3x9710xe2x88x923 type products resist solubilization in water but allow gradual discharge of the host SrCrO4 by dissolution. In contact with water, with no apparent effect on the dissolution rate, they reduce cisat, the solubility of host SrCrO4, as previously mentioned. xcex94cisat, the extent of reduction of the host""s solubility appears to be directly proportional to the degree of coverage of the host""s surface by the Srx/2PyOz layers. It seems to be dependent on the chemical identity of latter and notably, is maximized at saturation. xcex94cisat can be conveniently defined by: %xcex94ciast=100 (cisat-mdxe2x88x92cisat)/cisat, where cisat-md is the solubility of surface modified SrCrO4. Notably, %xcex94cisat values are  less than 0, by definition. It is interesting to note that xcex94cisat seems to be constant at saturation for each distinct (Srx/2PyO7)n/SrCrO4, n=1xc3x9710xe2x88x924-2xc3x9710xe2x88x923 type product. More importantly, however, xcex94cisat of a product appears to remain constant as the solubilization of the host SrCrO4 proceeds. Thus the discharge by dissolution of the host seems to resemble flow out of a xe2x80x9ccontainer. xe2x80x9d In the illustrative case of (Sr2P2O7)n/SrCrO4; n=2.2xc3x9710xe2x88x923, where the surface coverage is assumed to be complete at a thickness of 10-20 Angstrom, the host""s solubility is cisat-md=1.9 mmol/L and xcex94cisat=xe2x88x9263, as documented in 1.2 of Example 1.
Apparently, the desirable functional characteristics of surface modified pigment grade (Srx/2PyOz)n/SrCrO4 type products are maximized at n=1xc3x9710xe2x88x924 to 2xc3x9710xe2x88x923, that is at saturation of the host surface, which is preferred in the practice of the present invention. It can be concluded, that surface modified products of the present invention, behave like distinct chemical entities, having practically the chemical composition of technical grade SrCrO4 (with CrO42xe2x88x92, content undiminished) but with significantly reduced solubility in water.
The present invention provides an effective process for modifying the surface of finely divided pigment grade SrCrO4. As a consequence of surface modification, pursuant to the invention, the natural solubility of the host product is significantly reduced while the functional characteristics, important for pigment grade SrCrO4, are enhanced, inclusive of corrosion inhibitor performance and mitigation of clumping tendency.
Novel types of corrosion inhibitor pigment grade SrCrO4 are also provided, which can be generically symbolized by (Srx/2PyOz)SrCrO4, n=1xc3x9710xe2x88x924-2xc3x9710xe2x88x923. Here Srx/2PyOz symbolizes chemisorbed layers, as previously described. These novel type products behave like distinct chemical entities with respect to solubility, and generally, in pigment applications.
In practice, the surface modification procedure is realized by initially preparing an aqueous slurry of the host product, preferably of finely divided SrCrO4. This slurry can be freshly produced by precipitation of SrCrO4 or, alternatively, previously dried and finely ground product can be re-slurried in water, by intense agitation. Subsequently, the surface of the dispersed solid phase is modified, essentially according to Reaction 2 by chemisorption of diverse PyOzxxe2x88x92oxy anionic species of phosphorus, which result from dissociation of corresponding soluble salts or derivatives of the same. Practically, surface modification of SrCrO4 slurry is performed by introducing a soluble oxy salt of phosphorus into the aqueous phase and by stirring it, preferably for 15-30 minutes, at ambient or higher temperatures. With no intent to limit the concept of the present invention, the initial concentration of PyOzxxe2x88x92, realized in the aqueous phase, is preferred to be in the 0.5-1.5 mmole/L range and the PyOzxxe2x88x92/SrCrO4 molar ratio to be 1xc3x9710xe2x88x924-2xc3x9710xe2x88x923. The process is finalized by filtration and washing to soluble salt-free conditions and subsequent drying of the solid phase at 100-110xc2x0 C. The formed chemisorbed layer is also stabilized by the these steps. Typically, dry pigment grade products are ground to 100% 325 mesh fineness.
The oxy derivatives of phosphorous, applicable according to the present invention, include neutral or acidic Li+, Na+, K+, NH4+salts, and in general, any of the salts of: ortho-, pyro-, tri- and ploy-phosphoric acids, meta-phosphoric, poly-metaphosphoric, cyclic trimetaphosphoric acid, and phosphorous acids as well as mixtures thereof. It will be apparent that the list of applicable derivatives also includes the corresponding acids, the relatively soluble acidic salts of Ca2+, Sr2+, Ba2+, Mg2+, as well as the alkyl- or aryl-ammonium salts of the same. Notably, various acidic organic esters, primarily of ortho-, pyro-, or tri-phosphoric acid, phosphonic acid, as well as diverse phosphate-functionalized polymers and soluble salts of all the above, are also useful.
Examples of phosphorus oxy compounds, preferred in the practice of the invention for surface modification include: Na3PO4, Na2HPO4 and more preferably Na4P2O7, Na5P3O10 as well as the corresponding acids.
Examples of other applicable compounds include: Ca(H2PO4)2 .H2O, CaH2P2O7, (NaOPO2)3, Na3H15Al2 (PO4)8 and NaH14Al3(PO4)84H2O (dubbed as SALP-s in the bakery industry), soluble Na-polymetaphosphate, salts of hetero-poly acids, i.e.: H3PW12O40, H3PMo12O40; chelating agents and salts: phosphorus derivatives of nitrilotriacetic acid, (HO (O) CH2)2xe2x80x94NCH2xe2x80x94CH2xe2x80x94PO3H2 and alternatives such as N(CH2PO3H2) or nitrilotris(methylene phosphonic acid) and 1-hydroxyethylidene-1,1-diphosphonic acid, dubbed as HEDPA.
Also useful are salts of alkyl acid phosphates, (RO)2P(O)OH, (RO)P(O)(OH)2, where Rxe2x80x94is an alkyl-, aryl- or poly-ethoxylated derivative. Finally, with no intent to limit the concept, however, phosphorilated starch, [(NaO)2P(O)xe2x80x94Oxe2x80x94CH2xe2x80x94C6(O)H3(OH)2xe2x80x94Oxe2x80x94]n, is presented as an example of a phosphate-functionalized polymer.
Notably, it was also observed pursuant to the present invention, that a similar effect is achievable on other chromates, as well. More specifically, it was experimentally proven that the same procedure, ie., the chemosorption of diverse derivatives of the phosphoric acids, is very effective also in reducing the natural solubility of calcium and Zn(II) chromates.